25CrMnMo钢表面缺陷成因分析及控制

ANALYSIS AND CONTROL OF SURFACE DEFECTS OF 25CrMnMo STEEL

通过宏观形貌观察、微观组织(SEM)和能谱分析(EDS)等手段,分析了25CrMnMo钢管生产过程中表面缺陷产生的原因,并进行了系统优化。通过对25CrMnMo钢的理化性能分析,发现钢管表面宏观缺陷存在结晶器保护渣剥落现象,而微观分析发现钢管皮下含有许多大尺寸FeO-Na2O-Al2O3-CaO不规则夹杂物。同时,将钢管表面缺陷处夹杂物成分与长水口、浸入式水口以及炉渣成分、结晶器内保护渣进行对比分析,发现钢内夹杂物中Na、Al为结晶器保护渣的代表元素,推断原工艺条件下,卷入且来不及上浮的结晶器保护渣在凝固前沿时被凝固坯壳捕获,并形成表面及皮下夹杂缺陷是25CrMnMo钢管出现缺陷的主要原因。通过将浸入式水口的插入深度由80~130 mm改为90~130 mm,电磁搅拌电流由300 A降为200 A,频率3.0 Hz不变等降低卷渣指数的措施,有效改善了25CrMnMo钢结晶器卷渣问题,提高了铸坯质量。

The causes of surface defects in 25CrMnMo steel pipe production were analyzed by means of macroscopic morphology observation,microstructure(SEM)and energy spectrum analysis(EDS).Through the analysis of the physical and chemical properties of 25CrMnMo steel,it is found that the macro-defects on the surface of the steel pipe exist the spalling phenomenon of mold powder,while the micro-analysis shows that there are many large-sized irregular inclusions of FeO-Na_2O-Al_2O_3-CaO under the steel pipe skin.At the same time,the inclusion composition in the surface defect of steel tube is compared with that of long nozzle,submerged nozzle,slag composition and mold flux,it is found that Na and Al in the inclusions in steel are the representative elements of mold powder,and it is inferred that the mold powder which is involved and can not rise in time is captured by the solidified shell at the solidification front under the original technological conditions,surface and subcutaneous inclusion defects are the main causes of defects in 25CrMnMo steel pipe.By changing the insertion depth of the submerged nozzle from 80~130 mm to 90~130 mm,reducing the electromagnetic stirring current from 300 A to 200 A,and keeping the frequency 3.0 Hz constant,the slag entrapment index can be reduced,the problem of slag entrapment in mould of 25CrMnMo steel was effectively improved,and the quality of billet was improved.